This invention relates to a chuck assembly for chucking, during lathing, a workpiece, especially a workpiece which is thin and brittle and thus is likely to get warped when chucked.
A chuck assembly suitable for chucking a thin and brittle workpiece is disclosed in unexamined Japanese patent publication 7-256505.
FIGS. 8 and 9 show such a chuck assembly, which includes a tubular chuck body 51 concentrically mounted on a lathe main spindle, and a collet chuck 52 concentrically mounted on the chuck body 51 for gripping a centering reference portion of a workpiece W. The chuck body 51 is formed with a plurality of small cylindrical holes 53 arranged on a plane perpendicular to the axis of the chuck body at equal angular intervals with their axes extending radially of the chuck body. A piston P is slidably received in each cylindrical hole 53 to form a cylinder.
Oil is supplied into the cylinders C through oil passages 54 formed in the chuck body 51 to protrude rods L from the inner periphery of the chuck body 51 by pushing the pistons P. When oil pressure is released, the rods L are retracted by unillustrated springs mounted in the pistons P.
To chuck a workpiece with this chuck assembly, the workpiece W is put on the inner surface of the chuck body 51, and its centering reference portion is chucked by the collet chuck 52 to center the workpiece. In this state, the workpiece W is chucked by pressing the rods L of the cylinders C against the outer surface of the workpiece W.
This conventional chuck assembly chucks the workpiece with a plurality of cylinders C arranged at equal angular intervals along the circumference concentric with the main spindle. If the workpiece is a thin, brittle member, weak oil pressure is applied to the cylinders C. Thus, if there is even slight roughness on the surface of the workpiece, the pistons tend to move to different degrees from each other. Thus, this device has no self-centering function.
In this arrangement, a workpiece has to be centered by chucking its centering reference portion with the collet chuck 52 before being chucked by the plurality of cylinders C. Thus, for workpieces having different centering reference portions, different collet chucks are needed. That is, different chucks are needed for different kinds of workpieces W.
When a guide is used to center a workpiece, not so accurate centering is needed for rough machining because in rough machining, the machining depth is large. But for finish machining, if a large clearance for inserting the guide is provided to cope with various kinds of workpieces, the clearance tends to be too large if the workpiece is small. Too large a clearance makes it difficult to center the workpiece with high accuracy.
An object of this invention is to provide a chuck device which is simple in structure and can accurately and rigidly chuck various kinds of thin and brittle workpieces.